13.11 Ericoid endomycorrhizas

Most plants belonging to the order Ericales are able to associate
symbiotically with soil fungi to form the distinctive ericoid mycorrhiza.
This association was initially described in members of the family Ericaceae,
which is more abundant in the northern hemisphere and is probably best known in
the UK through heather moorland (‘heathland’) genera characterised by Erica
(heather), Calluna (ling) and Vaccinium (bilberry), though the
group also includes Rhododendron. These are plants that endure
moorlands and similar challenging environments, as heathland habitats are
typically found at high altitudes and colder latitudes, and have nutrient poor,
acidic soils. A morphologically similar mycorrhizal association occurs in the
family Epacridaceae, which is widely distributed in the southern hemisphere,
particularly southern Australia; the family takes its name from the genus
Epacris.

The fungi involved in ericoid associations are members of the Ascomycota.
Ericoid fungal hyphae form a loose network over the hair root surface (Fig. 9A).
The hyphae also penetrate the epidermal cells, often at several points in each
cell (Fig. 9B, C), and those cells become filled with coils of hyphae. As in all
endosymbioses, the intracellular fungal symbiont is separated from the plant
cytoplasm by a plant-derived membrane, which invaginates to
follow fungal growth and coil formation. Up to 80% of the hair root volume can
be comprised of fungal tissue. It is through these coils that nutrient
exchange occurs. This colonisation by the fungus is limited to mature and fully
expanded epidermal cells. In the apical region of the hair root, just behind the
growing meristem, the immature epidermis remains uncolonised.

Ericoid mycorrhizal fungi are facultative symbionts; that
is, they can exist as free-living mycelia in the soil, and they can also be
cultured in artificial media. When grown on nutrient agar the fungi produce
dark-coloured, slow growing and sterile mycelia. Absence of spores and
reproductive structures has complicated the identification and classification of
ericoid fungi and generated increasing use of molecular techniques for examining
DNA and RNA profiles. These show that there is considerable genetic diversity
between isolates that are superficially similar in appearance.
Comparative genomics and transcriptomics show that the fungal genes most highly
upregulated in the ericoid mycorrhizal symbiosis are those coding for fungal and
plant cell wall-degrading enzymes, lipases, proteases, and transporters. This
gene transcription pattern suggests a versatile dual saprotrophic and biotrophic
lifestyle quite like fungal endophytes (see
Section 13.19; Martino et al.,
2018).

Ericaceous shrubs can become dominant in many heathland communities,
especially in environmental conditions where plant litter is only slowly
decomposed, resulting in acidic soils rich in hard-to-digest organic matter but
low in available mineral nutrients such as nitrogen and phosphorus. These
mycorrhizas improve nitrogen and phosphorus
uptake by the plant, enabling the host plants to access otherwise unavailable
organic nutrients. The network of hyphae covering the root and the mycelium in
the soil digest polypeptides saprotrophically and absorbed nitrogen is exchanged
with the plant host. In extremely harsh conditions (for example winter in high
altitude and northern latitudes) the mycorrhiza may even support the host plant
with carbon nutrients (by metabolising polysaccharides and proteins for their
carbon content). Normally, though, the fungus takes photosynthetically-produced
carbohydrates from the plant host.

These physiological attributes of the mycorrhizal fungus enable ericaceous
plants to act as pioneer species, colonising unpromising habitats ranging from
arid sandy soils in the southern hemisphere to moist mor humus (a raw humus
state of unincorporated organic material, poorly mineralised and with an acid
pH; only slightly more degraded than peat) of the northern hemisphere; and the
absence of ectomycorrhizal fungi seems to prevent colonisation of heathland by
tree species like pines and birch (Collier & Bidartondo, 2009). Also thanks to
the mycorrhizal fungi, ericaceous plants are able to grow in highly polluted
environments, where the soil contains otherwise toxic levels of metal ions.
Taken together, the mycorrhizal symbiosis enables ericaceous plants to survive
under nutrient stressed conditions and improves the ability of
the host to compete successfully with other plants even on polluted sites
contaminated by heavy metals (Mitchell & Gibson, 2006; Perotto, Girlanda &
Martino, 2002).